Experience has shown that present films, particularly 35-mm photographic films, have reached such a level of sensitivity that they are adversely affected by low level protrusions which may be present on the surface of the winding core. Such protrusions may result from the winding of the narrowed portion of the web which has been attached to the spool. As the film is wound, this narrowed portion emerges from the slot, having its first convolution adjacent the spool. As the succeeding wider film is wound over the narrowed convolution, usually at high speeds and pressure, the surfaces on which these successive widths of film overlay are often uneven because of the previously wound narrowed portion of the film adapted to be secured to the spool. This condition results in a finished film having pressure marks, caused by such surface unevenness and the subsequent difference in pressure on the succeeding convolutions wound around the uneven surface at the spool core.
Other protrusions may occur as a result of the injection-molding process for producing an integral film spool as a unitary element. These protrusions can result from flashing imperfections which occur at the parting line of the mold. Such flashing defects may rise no higher than about 0.005 inch above the core surface; however, such defects are sufficient to cause pressure marks on subsequently wound film. In addition, imperfections can occur as a result of scratches on the core surface which occur during the handling of the spool prior to the film-winding operation.
Previous attempts to eliminate such pressure marks from films of the type having a narrowed trailing end attached to a slotted spool have not been entirely successful. They have relied upon, for example, a grooved central portion of the core which attempts to compensate for the thickness and length of the wound narrowed trailing end of the film. Such spools are incapable of solving the pressure marks resulting from spools manufactured by an injection-molding process, or imperfections to the spool-core surface which are received prior to the winding and packaging of the highly sensitive film.
It is the object of the present invention to provide a slotted spool which, when manufactured as a unitary part by means of an injection-molding process, provides a smooth, even supporting surface for convolutions of the wound film, thereby eliminating pressure marks on the film.